Method for improving the torsional fatigue strength of crankshafts



May 12, 1970 G. SQEL-JLEN ET AL 3,5 11 ,928

METHOD FOR IMPROVING THE TORSIONAL FATIGUE STRENGTH OF CRANKSHAFTS Filed Feb. 12, 1968 2 Sheets-Sheet 1 Fig. 2

May 12, 1970 G. SEULEN ET AL METHOD FOR IMPROVING THE TORSIONAL FATIGUE 2 Sheets-Sheet 2 STRENGTH OF 'GRANKSHAFTS Filed Feb. 12, 1968 |lIlIlIlllllllllllllllllllllllllll llllllllllllIllllllllllllllllll Fig. 4

Z z lngenga/S United States Patent 3,510,928 METHOD FOR IMPROVING THE TORSIONAL FATIGUE STRENGTH OF CRANKSHAFTS Gerhard Seulen and Friedhelm Reinke, Remscheid, Germany, assignors to AEG-Elotherm G.m.b.H., Remscheid-Hasten, Germany Filed Feb. 12, 1968, Ser. No. 704,891 Claims priority, application Germany, Oct. 6, 1967, A 57 003 Int. Cl. B21d 53 10,-B21k l/08 US. Cl. 29149.5 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the inductive hardening of crankshafts, and particularly relates to the improvement of the torsional fatigue strength of inductively hardened crankshafts.

For improving the wear resistance of crankshafts it has previously been the practice to provide the bearing surfaces thereof, i.e. the crankpins and main bearing surfaces, with a hardened layer produced almost exclusively by an inductive hardening process.

Inductively hardened crankshafts have been proposed in which the hardened layers are not confined to the surfaces principally exposed to abrasive wear, but which also extend into the angles between the bearing surfaces and the crank webs, and further into the axial abutment faces. Such hardened layers impart a considerable degree of bending fatigue strength to crankshafts thus treated.

Crankshafts which have been inductively hardened in this way have proved to be very satisfactory in practice. By thus hardening crankshafts without modifying them in any other way it has been possible to use them in engines developing much higher power than would have been possible when ordinarily hardened crankshafts are used. The hardening of the angles at the webs is capable of improving the bending fatigue strength of crankshafts by about 80 to 100%.

It has now been found that when overloads arise in service, the most critical part of a crankshaft that has this improved bending fatigue strength is the region surrounding the oil holes in the main bearings and crankpins. If the static and dynamic loads exceed a critical level torsional fracture first occurs at these points, possibly because of the reduced volume of material left in the region of the oil holes.

So far this tendency tofailure of crankshafts has not successfully been countered. Attempts have been made to eliminate the causes of torsional fatigue fracture by improving the surface quality at the edges of the oil holes or by subsequent work hardening. However the expense of carrying out this method in mass production for achieving the desired results, is too high. Attempts at solving the problem by using high alloyed steels have also failed because the cost of the product becomes uneconomically high due to the higher cost of materials and expensive machining.

The object of the invention is to improve the torsional fatigue strength of low alloyed or unalloyed steel crankshafts which have been inductively hardened to improve the torsional fatigue strength thereof.

3,510,928 Patented May 12, 1970 The invention consists of a method of improving the torsional fatigue strength of crankshafts having an oil hole in a main bearing and/ or crankpin comprising inductively heating the bearing surface extending at least from the angle between the bearing surface and the crankweb to provide an inductively hardened surface layer which progressively increases in depth towards the said oil hole.

According to a preferred feature of the invention, the depth of the surface hardened layer increases in depth towards the oil hole in such a way that the inside surface of the entire length of the oil hole is provided with a hardened surface layer. It may be preferred however inductively to harden the interior of the oil hole to a distance of about half the diameter of the shaft or to a distance of less than half the diameter of the bearing surface.

For performing the method according to the invention apparatus may be used comprising two inductors which take effect consecutively during the hardening of the crankshaft. Both heating conductor loops or inductors are independently controllable with respect to power, frequency and heating time.

Embodiments of the invention are hereinafter described and illustrated in FIGS. 1 to 4 of the accompanying drawings, of which FIGS. 1 and 2 each show the crankpin of a crankshaft depths,

FIG. 3 is a schematic representation of apparatus for carrying out the method according to the invention, and

FIG. 4 is a representation in graphic form of the electrical power supplied to the inductors of the apparatus of FIG. 3, against time.

Referring to FIG. 1, a part of a crankshaft includes a crankpin 1 and crank webs 2. On each side of the crank the shaft extends to form main bearing surfaces 3. Each bearing, i.e. the crankpin and the main bearing of the shaft, is provided with oil holes 4 in the form of bores, which may extend diametrically as shown in FIG. 1 or at an angle, through the bearing. The boundary between the inductively hardened and unhardened material is indicated in FIG. 1 by a discontinuous line, the zone of hardening being indicated by 5. The hardened zone 5 at each end of the crankpin in the abutment zone embraces the angle between the pin and the web and then progressively increases in thickness towards the oil hole 4. The hardened zone 5 in FIG. 1 on the surface of the crankpin completely penetrates the crankpin in the neighbourhood of the oil hole. Consequently the entire inside surface of the oil hole is surface-hardened. 7

FIG. 2 shows an alternative degree of hardening, the zone of hardening being likewise indicated at S, which progressively increases in depth towards the oil hole 4, but in the neighbourhood of the oil hole the depth of penetration into the crankpin is not so great as that shown in FIG. 1. The internal surface of the oil hole is not therefore surface-hardened throughout its entire length. Apparatus according to the invention for improving the torsional fatigue strength of a crankshaft having an oil hole in a main bearing portion and/or a crankpin portion of the shaft for the lubrication of the bearing surface thereof, comprises two inductor elements adapted to produce heating zones of different width in the direction of the axis of the said main bearing portion, and means for consecutively energising the said inductor elements for hardening the crankshaft.

One embodiment of apparatus for carrying out the method of hardening according to the invention is schematically shown in FIG. 3. Two inductors or two heating conductor loops of one inductor, are located above the surface 6 that is to be hardened, the surface being shown developed in the plane 'of the paper. The heating zones produced by heating conductor loops or inductors 7 and 8 are different in width, due to their shape and configuration. The elements 7 and 8 may be described as inductors if they are mechanically separate. However, heating con ductors which are supplied with different electrical powers, at different frequencies at different times may very well be contained in one inductor casing. In the following description it is therefore assumed for the sake of simplicity that 7 and 8 are merely heating conductor loops, although two separate inductors might just as well be used.

Thus the two heating conductor loops 7 and 8 are each connected to an electrical medium frequency generator through a switch 11 and 12 respectively. However, normally the heating conductor loops are connected to different electrical generators which may differ more particularly in the frequency they supply. The said inductors are placed adjacent the portion of the crankshaft to be heated, and the crankshaft rotated so that relative movement occurs between the crankshaft and the conductor loops 7 and 8.

The region surrounding the oil hole 4 is heated and after the surface has reached hardening temperature heat ing is continued to maintain the same temperature level until a heating zone of the desired depth has been produced. Isothermal heating may partly be provided by the second inductor although the principal purpose of this inductor is to heat up the entire bearing surface, possibly including the angles at the webs. The heating times are so displaced that the workpiece can be quenched from a common temperature level.

The process of inductive heating of the surface that is to be hardened is intended to be so performed that the first heating conductor loop 7 is first energised by the generator G at lower power N for a period t as indicated in the graph in FIG. 4, which the ordinate represents power N supplied to the coils and the abscissa represents time t. At the end of the time 23 the switch 11 is opened and the switch 12 closed. The heating conductor loop 8 is now supplied by the generator G with a power N for a period t If the power, frequency, time and shape of the heating conductor loops are correctly 4 chosen, the bearing surface will have been inductively heated at the end of the total period t +t in the manner illustrated in FIG. 1 or FIG. 2 so that hardening can be accomplished by quenching with a liquid or gaseous medium.

Satisfactory results will be achieved if the heating conductor loop 7 is supplied with electrical energy of lower frequency. It is nevertheless possible to supply both heating conductor loops from the same electrical generator, particularly as a result of the recent availability of high power inverters using thyristors whereby the frequency of the supply voltage can be more easily adjusted within limits to different values, compared to the use of conventional static or dynamic medium frequency generators.

What is claimed is:

1. A method of improving the torsional fatigue strength of a crankshaft having an oil-hole in a main bearing portion and/or crankpin portion of the shaft for the lubrication of the bearing furface thereof, which comprises inductively heating and hardening the said bearing surface so as to produce a hardened surface layer thereon which progressively increases in depth towards the said oil-hole.

2. The method according to claim 1, which comprises heating the bearing surface to a depth towards the oilhole whereby the internal surface of the oil-hole in the hardened shaft is hardened along its entire length.

3. The method according to claim 1, in which the internal surface layer of the oil-hole is inductively hardened to a distance that is less than half the diameter of the bearing surface.

References Cited UNITED STATES PATENTS THOMAS H. EAGER, Primary Examiner US. Cl. X.R. 296 

